A conventional boiling water reactor (BWR) is shown in FIG. 1. Feedwater is admitted into a reactor pressure vessel 10 via a feedwater inlet 12 and a feedwater sparger 14, which is a ring-shaped pipe having suitable apertures for circumferentially distributing the feedwater inside the reactor pressure vessel (RPV). The feedwater from sparger 14 flows downwardly through the downcomer annulus 16, which is an annular region between RPV 10 and core shroud 18. In addition, a core spray inlet 11 supplies water to a core spray sparger 13 (located inside the shroud 18) via core spray header 15, core spray downcomer piping 17 and core spray elbow 19 (which penetrates the shroud wall). The core spray header 15 has a circular section that occupies space directly underneath feedwater sparger 14.
Core shroud 18 is a stainless steel cylinder surrounding the nuclear fuel core. The core is made up of a plurality of fuel bundle assemblies 22 (only two 2.times.2 arrays of which are shown in FIG. 1). Each array of fuel bundle assemblies is supported at the top by a top guide 20 and at the bottom by a core plate 21. The core top guide 20 provides lateral support for the top of the fuel assemblies and maintains the correct fuel channel spacing to permit control rod insertion.
The water flows through downcomer annulus 16 to the core lower plenum 24. The water subsequently enters the fuel assemblies 22, wherein a boiling boundary layer is established. A mixture of water and steam enters core upper plenum 26 under shroud head 28. Vertical standpipes 30 atop shroud head 28 are in fluid communication with core upper plenum 26. The steam-water mixture flows through standpipes 30 and enters steam separators 32, which are of the axial-flow centrifugal type. The separated liquid water then mixes with feedwater in the mixing plenum 33, which mixture then returns to the core via the downcomer annulus. The steam passes through steam dryers 34 and enters steam dome 36. The steam is conducted from the RPV via steam outlet 38.
The BWR also includes a coolant recirculation system which provides the forced convection flow through the core necessary to attain the required power density. A portion of the water is pumped from the lower end of the downcomer annulus 16 via recirculation water outlet 42 and forced by a centrifugal recirculation pump (not shown) into jet pump assemblies 44 (only one of which is shown) via recirculation water inlets 46. The BWR has two recirculation pumps, each of which provides the driving flow for a plurality of jet pump assemblies. The jet pump assemblies are circumferentially distributed around the core shroud 18.
The core shroud 18 (shown in more detail in FIG. 2) in one type of BWR comprises a shroud head flange 18a for supporting the shroud head 28; a circular cylindrical upper shroud wall 18b having a top end welded to shroud head flange 18a; an annular top guide support ring 18c welded to the bottom end of upper shroud wall 18b; a circular cylindrical middle shroud wall comprising three sections 18d, 18e and 18f welded in series, with a top end of section 18d being welded to top guide support ring 18c; and an annular core plate support ring 18g welded to the bottom end of middle shroud wall section 18f and to the top end of a lower shroud wall 18h. The entire shroud is supported by a shroud support 50, which is welded to the bottom of lower shroud wall 18h, and by annular shroud support plate 52, which is welded at its inner diameter to shroud support 50 and at its outer diameter to RPV 10.
In the event of a seismic disturbance, it is conceivable that the ground motion will be translated into lateral deflection relative to the reactor pressure vessel of those portions of the shroud located at elevations above shroud support plate 52. Such deflections would normally be limited by acceptably low stresses on the shroud and its weldments. However, if the shroud weld zones have failed due to stress corrosion cracking, there is the risk of misalignment and damage to the core and the control rod components, which would adversely affect control rod insertion and safe shutdown.
Stress corrosion cracking in the heat affected zone of any shroud girth seam welds diminishes the structural integrity of shroud 18, which vertically and horizontally supports the core top guide 20 and the shroud head 28. In particular, a cracked shroud increases the risks posed by a loss-of-coolant accident (LOCA). During a LOCA, the loss of coolant from RPV 10 produces a loss of pressure above the shroud head 28 and an increase in pressure inside the shroud 18, i.e., underneath shroud head 28. The result is an increased lifting force on shroud head 28 and on the upper portions of the shroud to which the shroud head is bolted. If the core shroud has fully cracked girth welds, the lifting forces produced during a LOCA could cause the shroud to separate along the areas of cracking, producing undesirable leaking of reactor coolant.
A known repair method for vertically restraining a weakened core shroud utilizes tensioned tie rods 54 coupled to the shroud flange 18a and to the shroud support plate 52, as seen in FIG. 2. The lower end of the tie rod/lower spring assembly hooks underneath a clevis pin 60 inserted in a hole machined into gusset plate 58, which plate is in turn welded to shroud support plate 52 and RPV 10. In addition, the shroud 18 is restrained laterally by installation of wishbone springs 56a/56b and 72, which are components of the shroud repair assembly.
Referring to FIG. 2, the shroud restraint tie rod/lower spring assembly comprises a tie rod 54 having a circular cross section. A lower end of tie rod 54 is anchored in a threaded bore formed in the end of a spring arm 56a of a lower spring 56. Tie rod 54 extends from the end of spring arm 56a to a position adjacent the outer circumferential surface of the top guide support ring 18c. The upper end of tie rod 54 has a threaded portion.
The lower spring 56 is anchored to a gusset plate 58 attached to the shroud support plate 52. The lower spring 56 has a slotted end which straddles gusset plate 58 and forms a clevis hook 56c. The clevis hooks under opposite ends of a clevis pin 60 inserted through a hole machined in the gusset plate 58. Engagement of the slotted end with the gusset plate 58 maintains alignment of lower spring 56 under the action of seismic motion of the shroud, which may be oblique to the spring's radial orientation.
The tie rod 54 is supported at its top end by an upper support assembly 62 which hangs on the shroud flange 18a. A pair of notches or slots are machined in the shroud head ring 28a of shroud head 28. The notches are positioned in alignment with a pair of bolted upper support plate segments 64 of upper support assembly 62 when the shroud head 28 is properly seated on the top surface of shroud flange 18a. These notches facilitate coupling of the tie rod/lower spring assembly to the shroud flange.
The pair of notches at each tie rod azimuthal position receive respective hook portions 64a of the upper support plates 64. Each hook 64a conforms to the shape of the top surface of shroud flange 18a and the shape of the steam dam 29. The distal end of hook 64a hooks on the inner circumference of shroud dam 29.
The upper support plates 64 are connected in parallel by a top support bracket (not shown) and a support block 66 which forms the anchor point for the top of the tie rod. Support block 66 has an unthreaded bore, tapered at both ends, which receives the upper end of tie rod 54. After the upper end of tie rod 54 is passed through the bore, a threaded tensioning nut 70 is screwed onto the upper threaded portion 54a (see FIG. 4) of tie rod 54.
As seen in FIG. 2, the assembly comprised of support plates 64 with hooks 64a, support block 66, tie rod 54, lower spring 56, clevis pin 60 and gusset plate 58 form a vertical load path by which the shroud flange 18a is connected to the shroud support plate 52. In the tensioned state, the upper support plates 64 exert a restraining force on the top surface of shroud flange 18a which opposes separation of the shroud 18 at any assumed failed circumferential weld location.
Lateral restraint at the elevation of the top guide support ring 18c is provided by an upper spring 72 having a double cantilever "wishbone" design. The end of the radially outer arm of upper spring 72 has an upper contact spacer 74 rotatably mounted thereon which bears against the inner surface of the wall of RPV 10.
A spring arm 56a of lower spring 56 laterally supports the shroud 18 at the core plate support ring 18g, against the vessel 10, via a lower contact spacer 76. The top end of spring arm 56a has a threaded bore to provide the attachment for the threaded bottom end 54b (see FIG. 4) of tie rod 54. The member 56d connecting the upper wishbone spring 56a, 56b to clevis hook 56c is offset from the line of action between the lower end of tie rod 54 and clevis pin 60 to provide a vertical spring compliance in the load path to the tie rod.
A middle support 80 is preloaded against the vessel wall at assembly by radial interference which bends the tie rod 54, thereby providing improved resistance to vibratory excitation failure of the tie rod. The middle support also provides a lateral motion limit stop for the shroud central shell, in the event of complete failure of its girth welds. To facilitate mounting of the middle support 80, a mid-support ring 82 is secured to the tie rod 54, as shown in FIG. 4. The middle support 80 has a section of an annular recess counterbored in its bottom which form fits on ring 82, thereby preventing lateral shifting of middle support 80 relative to tie rod 54. The middle support 80 is latched to midsupport ring 82 by a wishbone spring latch (not shown), which blocks upward vertical displacement of middle support 80 relative to tie rod 54.
During installation of the shroud repair hardware shown in FIG. 2, the tie rod/lower spring assembly comprising tie rod 54 screwed into lower spring 56 is suspended from a cable and lowered into the annulus to the desired elevation. Only after clevis hook 56c has been hooked under clevis pin 60 and the tie rod/lower spring assembly has been braced in the hooked position will the upper support assembly 62 be installed, followed by upper spring 72.
As the cable is lowered, the tie rod/lower spring assembly must be guided into the narrow space between adjacent jet pump assemblies. However, in some BWRs this installation site lies below the feedwater sparger, core spray header and core spray downcomer piping, which lie in the path of a descending tie rod suspended from an overhead crane. To protect the feedwater sparger and core spray header from damage due to impact by the descending tie rod/lower spring assembly, which weighs in excess of 1,000 pounds, a cover is hooked onto the feedwater sparger to deflect the tie rod away from the feedwater sparger and core spray header. However, the cover obstructs the cable so that the tie rod/lower spring assembly does not hang plumb from the crane. This makes it difficult to maneuver a suspended tie rod/lower spring assembly into the correct position in the downcomer annulus. In particular, unless appropriate steps are taken, the cover will obstruct the taut cable from becoming oriented vertical and limit radially outward movement of the cable at the point of contact and tie rod/lower spring assembly suspended therefrom. Also the friction between the taut cable and the cover impedes tangential movement of the suspended tie rod/lower spring assembly. As a result, the azimuthal and radial positions of the tie rod/lower spring assembly cannot be controlled by moving the crane to a corresponding position overhead, preventing placement of the suspended tie rod/lower spring assembly at the precise position required for coupling to the gusset plate.